It sounds like an engine misfire is occuring. Does the car go in to the lower gear and eventually accelerate? It sounds like your vehicle is misfiring. When you put a greater load on in a higher gear, it takes all the that ignition coils can deliver ( the SHO motor has DIS or coil pack ignition )I worked on the SHO manual transmission equipped Taurus when they were new at the dealer. They should have a lot of power. When was the last time you replaced your wires, plugs etc?